Riding high from his big win in the Florida primary, Mitt Romney tried yesterday to deflect criticism that the race for the Republican Party had become destructively negative saying it was good training for the "vitriolic, spiteful campaign" that will be unleashed later this year by President Barack Obama.

Mr Romney, who beat his closest rival, Newt Gingrich, by nearly 15 points in Florida and looks set to capture Nevada on Saturday, is once again the easy front-runner in the nomination derby. However, exit polling in Florida showed that he is still not winning the hearts of the party's conservative wing, and Mr Gingrich is vowing to stay in the fight.

The grinding negativity of the Florida race and new disclosures about the influence of independent fundraising groups known as political actions committees – or super PACs – are leaving a bad taste with some party activists. New filings to the Federal Election Commission reveal that the group supporting Mitt Romney, Restore Our Future, raised $30m (£19m) last year and still has almost $24m on hand to spend.

"Super PACs have fundamentally changed the way campaigns are run, and it's had a huge effect on the race," said Saul Anunzis, the former Michigan Republican Party chairman. "If you can find one donor who is willing to play in a big way, it can have an unbelievable impact." Mr Gingrich would probably have been forced out of contention but for donations of $10m to his Super PAC from one Las Vegas billionaire.

If Mr Romney does become the nominee, he will turn to his super PAC helpers to neutralise the fundraising prowess of President Obama, whose own PAC, Priorities USA, raised only $4.2m last year.

It emerged from the filings yesterday that Steven Spielberg has contributed large sums to Priorities USA, while Harold Simmons, a billionaire Texas banker, gave the Gingrich group $500,000. Mr Romney's group is being fuelled largely by big names in the investment industry. Six of these supporters gave the Romney PAC $1m apiece.

Special protection: Romney's rally drive

Mitt Romney has been granted Secret Service protection "because of the increase in crowd sizes" at his rallies, say campaign staff. The move prompted speculation on Twitter over his possible code name. "Dry Toast" (a reference to blandness) and "Mom's jeans" (relating to his attire) topped the tongue-in-cheek list.